Different technologies have been proposed in the past directed to the issue of coating titanium dioxide onto a support layer, for example pyrogenic techniques. For example, US 20030037705 discloses a titanium dioxide powder which contains iron oxide, which is obtained by flame hydrolysis of FeCl3 and TiCl4. However, this results in a mixture of the two oxides, and no indication of any crystalline phase is disclosed. JP 2004231927 teaches the deposition of Ti2O3 on silica sol particles by co-dispersing silica sol and dititanium trioxide particles obtained by firing a mixture of titanium hydride and titanium dioxide in an inert atmosphere.
Furthermore, electrochemical processes have been proposed for coating titanium dioxide onto support layers, for example in “Formation of nanoporous titanium oxide films on silicon substrates using an anodization process”, Yu et al., Nanotechnology, 2006, 17, 808-814, a porous film of titanium dioxide is deposited on a silicon substrate by electrochemical processes.
Most of the publications deal with hydrolytic techniques. For example, US2004120884 teaches the coating of carrier particles such as organic polymers with a sol of titanium dioxide particles obtained after several process steps. No crystalline phase of the final coating is disclosed in this publication. Furthermore, U.S. Pat. No. 5,837,050 discloses a process to make a sol of iron containing rutile crystallite, where the final material is a solution of rutile particles coated with iron oxide. U.S. Pat. No. 6,566,300 teaches the preparation of a photocatalyst by impregnation of zeolite ZSM-5 with a solution of titanium tetraisopropoxide. Another coating process is described in JP 2004161978, where a pigment particle, like an inorganic substance, is coated with titanium dioxide, and the coated pigment is then calcined. No indication of a crystalline titanium dioxide phase is given. Yamabi et al., Chem. Mater., 2002, 14, 609-614 describe the precipitation of titanium dioxide from solution at a pH below 2 at room temperature and an anatase or rutile phase may be obtained, depending on the pH and titanium (IV) concentration. However, barium chloride need to be added as an ion exchanger and the solution must be left at 60° C. for several days. Penpolcharoen et al., J. Adv. Oxide. Technol., 2002, 5, 1 describe a sol/gel method to coat nano-hematite and colloidal hematite particles with titanium dioxide. In the final step of the process, the material is calcined at 873K. Both of the titanium dioxide phases are obtained, but no control of the relative proportions of the phases is achieved. EP 0 282 329 B1 discloses flaky micaceous iron oxide which is coated hydrothermally with rutile type titanium dioxide in the presence of salts of Zn, Sn, Sb, Li. The coating layer needs to be calcined at a temperature ranging from 500° C. to 1000° C. Yin et al., J. Mater. Chem. 2001, 11, 1694-1703 disclose an amorphous titanium dioxide suspension which by hydrothermal treatment under acidic condition leads to the formation of a mixture of anatase, brookite and rutile nanocrystallites. The process disclosed in this documents involves autoclaving at 150° C. for 21 h.
Gennari and Pasquevich, J. Material Sci., 1998, 22, 1571-1578 describe a process in which a physical mixture of anatase, rutile (95% anatase) and α-Fe2O3 is heated at a temperature above 400° C. to study the kinetic of TiO2 phase transformation. The presence of iron oxide enhances conversion of anatase to rutile because Fe3+ ions that diffuse inside the TiO2 crystals allow formation of oxygen vacancies. Sato et all, J. Material Sci., 2006, 41, 1433-1438 describe a homogeneous precipitation of N-doped TiO2 from an organic solution of TiCl3. The material needs calcinations to convert into crystalline nitrogen-doped TiO2. The phase composition depends on precipitation solution pH and solvent changing from anatase to rutile or brookite.
When looking at the prior art, it is obvious that it is difficult to obtain titanium dioxide which is uniformly layered on a support. Many of the cited publications do not achieve a crystal phase at all, and some of those which disclose a process resulting in one or may be the other crystalline titanium dioxide phase cannot guarantee the proportions of the crystal phase composition. Thus, there is a need for a process by which the titanium dioxide crystal phase composition could be controlled by changing some of the operative conditions.
Another need is to deposit titanium dioxide with a nanocrystalline structure. Furthermore, many of the processes in the prior art turn out to be rather costly and elaborate, for example they include calcination or autoclaving steps. Thus, there is a further need for a process for deposition of titanium dioxide on a support which is simple and cost-efficient.